The present invention relates generally to improved methods of evaluating the performance of acidizing operations or treatments; and more specifically relates to improved methods for evaluating matrix acidizing operations for facilitating the determination of formation skin factor as a function of time during the conduct of the acidizing operation.
As is known in the industry, a well that is not producing as expected may be subjected to formation damage, and therefore may need stimulation to remove the damage and to increase the well's productivity. One type of treatment used to remove well damage is matrix acidizing. The purpose of matrix acidizing is to remove damage around the immediate area of the wellbore, thus increasing the well's productivity.
During matrix acidizing treatment, fluids are injected into the porous medium of the reservoir at low rates and pressures called "matrix" or "subfracturing" rates. In theory, the injected fluid dissolves some of the porous medium and all of the damaging material, thereby increasing the reservoir's permeability and productivity.
The degree of well damage is measured by the formation "skin factor". The skin factor is proportional to the steady-state pressure difference around a wellbore. A positive skin factor indicates that the well's flow is restricted, while a negative skin factor indicates flow enhancement, which is usually the result of stimulation. The skin factor is a multi-component measurement that takes into account a number of factors that may cause a restriction in well flow. The matrix acidizing process removes damage around the immediate area of the wellbore and thus reduces the part of the skin factor due to formation damage.
It would be desirable to evaluate the effectiveness of the matrix acidizing treatment in increasing a well's productivity. One conventional method of evaluating the effectiveness of a matrix acidizing treatment is to perform pre-treatment and post-treatment well tests. However, such a process is time consuming and expensive, and is not economically justified for most reservoirs.
Several attempts have been made to evaluate the effectiveness of matrix acidizing treatments by monitoring changes in the skin factor in real-time. The ability to monitor changes in skin factor as stimulation is performed helps evaluate whether an adequate fluid volume has been injected, indicates whether there is a need to modify the treatment, and helps to improve future well designs in similar situations.
One previous real-time evaluation method considers each stage of injection or shut-in during the treatment as a short, discrete well test. The transient reservoir pressure response to the injection of fluids is analyzed and interpreted to determine changes in the condition of the wellbore (skin factor) and the formation transmissibility. This method of using analysis of transient reservoir pressure is valid, however, only if the skin factor is not changing while a set of pressure data for one particular interpretation is being collected. However, injecting reactive fluids into the formation to remove damage causes the skin factor to change constantly during the operation thus rendering erroneous measurements. Hence, in order to be theoretically correct, this method requires the injection of a slug of inert fluid into the formation to generate the transient response for a constant skin factor each time the damage removal is assessed. The injection of inert fluid prior to each assessment is not practical and thus renders this method unworkable in the real world.
Another previous method uses instantaneous pressure and rate values to compute the skin factor at any given time during the treatment. The method, based on the steady-state, single-phase, radial version of Darcy's law, uses the concept of a finite radius "acid bank". This method relies on the assumption that the well is maintained at a "steady-state". This assumption may yield erroneous results since transient behavior is in effect for a time that greatly exceeds injection time. Thus, transient bottomhole pressure or unintentional changes in the injection rate are subject to being misconstrued as changes in skin factor.
A third prior art method involves using the rate history during a treatment and calculating the corresponding bottomhole pressure response for a constant value of skin factor. The difference between the simulated bottomhole pressure response and the bottomhole pressure response measured during the treatment is interpreted as resulting from the instantaneous pressure arising from the skin factor. The skin factor is calculated from this pressure difference and presented as a plot of skin factor versus time.
This evaluation method has several drawbacks. The major drawback is that the values of the well and reservoir parameters required for the simulated pressure response are not generally available. Thus, for matrix acidizing treatments an injection/falloff test must be performed prior to evaluation to obtain these values. Performing an injectivity/falloff test prior to the matrix acidizing treatment to determine permeability and skin factor from the falloff data analysis involves the added expense of additional fluid, pumping costs, and time. These added expenses may not be justified for small volume matrix acidizing treatments.
Additionally, for each incremental period, this computation method involves simulating a bottomhole pressure given the rate history up to that time, taking the difference between the calculated pressure and the measured pressure, and then calculating the observed skin factor, thus requiring more calculation steps than are necessary to generate a plot of skin factor versus time.
Accordingly, the present invention provides a novel matrix acidizing evaluation method which considers the effects of pumping rate variations, is fast, simple to implement, and can be performed in real-time. The method, therefore, provides a relatively quick and simple method for calculating formation skin factor during an acidizing operation.